In the related art, an optical amplifier is known that is called a Raman amplifier that amplifies signal light using an excitation light source. In the Raman amplifier, a phenomenon called stimulated Raman scattering is induced and thereby a gain value of the signal light is increased at a wavelength band including longer wavelengths than the center wavelength of the excitation light source by about 100 nm. Such amplification of signal light by the stimulated Raman scattering is called “Raman amplification”.
FIG. 18 is a diagram illustrating the Raman amplification. In FIG. 18, the horizontal axis indicates a wavelength, and the vertical axis indicates power. As illustrated in FIG. 18, in the Raman amplification, signal light of the wavelength band including a center wavelength that is away from a center wavelength of the excitation light source to the long-wavelength side by about 100 nm is amplified. Amplified stimulated Raman scattering (ASS) that is a noise component of the stimulated Raman scattering, which is generated in the Raman amplification, is relatively small.
In order to properly amplify signal light of a desired wavelength band using the Raman amplifier, it is desirable that shift of the center wavelength of the excitation light source that is used to perform Raman amplification for the signal light is reduced. In consideration of this, a Raman amplifier is known in which a wavelength locker (fiber Bragg grating: FBG) is provided that fixes the center wavelength of the excitation light source that emits the excitation light. In the Raman amplifier with the FBG, a gain equalizer (GEQ) is often provided on an output side of the Raman amplifier to equalize gains of signal light, for a desired wavelength band, that is pumped by the Raman amplifier with an excitation light source.
However, the center wavelength of the excitation light source, which is fixed by the FBG varies when the temperature of an ambient environment (hereinafter, referred to as “environment temperature”) such as a temperature of the FBG is changed. This is why the refraction index of an optical fiber included in the FBG has temperature dependence. When the center wavelength of the excitation light source, which is fixed by the FBG varies, the gain of the signal light that is pumped by the Raman amplifier with the excitation light source also varies. Consequently, it becomes difficult to equalize the gains by the GEQ, so that a case undesirably occurs in which it is difficult to maintain flatness of a profile of the gain deviation for the wavelengths.
As a technique to avoid such a case, a technique has been proposed that controls the temperature of the FBG to adjust the center wavelength of the excitation light source. In the technique, the FBG and an erbium-doped fiber (EDF) that is located downstream of the Raman amplifier are housed in a casing together, and the temperature of the FBG is adjusted by the temperature of the EDF. As a result, when the environment temperature is changed, shift of the center wavelength of the excitation light source, which is fixed by the FBG, may be reduced, and flatness of the profile of the gain deviation in the wavelength band may be maintained.
Japanese Laid-Open Patent Publication No. 2004-103599 is examples of the related art.
However, in the related art, it may remain difficult to accurately maintain flatness of the gain deviation profile in the wavelength band when the environment temperature is changed.
Specifically, in the related art, the temperature of the FBG is adjusted using the temperature change of the EDF, however, a temperature of the GEQ that is provided on the Raman amplifier output side is not considered when the temperature of the FBG is adjusted. The center wavelength of the wavelength band that is a target of gain equalization by the GEQ varies depending on the change of the environment temperature. Therefore, when the environment temperature is changed, the center wavelength of the excitation light source, which is fixed by the FBG, is not in accordance with the center wavelength of the wavelength band that is a target of gain equalization by the GEQ. As a result, in the related art, it may remain difficult to accurately maintain flatness of the gain deviation profile in the wavelength band.